1. Field of the Invention
The present invention relates to varistor disks. More specifically, the invention relates to a method and apparatus for mounting a varistor disk to the commutator of a motor without the need for soldering.
2. Related Art
As a motor runs, its brushes make and break contact with the commutator. The inductance of the motor windings prevents the current from changing instantly when the brushes break their connection. As a result, a large voltage is generated, creating sparks. These sparks have a lot of high frequency content, causing electro-magnetic interference (EMI).
Varistor disks are often used to suppress the EMI which can be generated by the brushes on a DC motor. A varistor has high resistance at low voltages and has lower resistance as the voltage increases. It is similar to two zener diodes connected back to back to provide the same effect regardless of the polarity of the voltage. In order to suppress the EMI, a varistor is usually soldered to the tangs of the commutator, thereby providing a varistor effect between adjacent commutator segments.
U.S. Pat. No. 4,859,893 (Wang) is illustrative of a commutator which uses a varistor to suppress EMI. The varistor is clamped onto the terminals of the commutator using a unitary plastic clamping member having a plurality of fingers which serve to protect the varistor ring from vibration which might cause the varistor to break.
British Patent No. 2,177,854A (Mabuchi) discloses the use of an electrically conductive rubber element to prevent EMI by suppressing sparks between motor brushes and respective commutator segments. The element is annular and is adhered to the terminal portions of the commutator by an electrically conductive adhesive.
Russian Patent No. 746790 (Selyaev) discloses the use of a dielectric ring to suppress sparking. The dielectric ring is bonded to the commutator using an epoxy which is coated on the non-metallized side of the ring.
Japanese Patent No. 29,906 (Morisawa) discloses the use of a variably conductive layer for reducing sparking between motor brushes and commutators.
Soldering of the varistor disk to the commutator tangs is a labor intensive operation not readily conducive to automation. Also, as successive parts of the varistor disk are heated during soldering, thermal stresses occur which can cause cracking of the varistor disk. Sometimes, there will be a cold solder joint, which will cause the loss of the suppression effect for the involved commutator segments. Although use of a conductive adhesive to adhere the varistor disk to the tangs avoids the thermal stresses resulting from soldering, it is only slightly less labor intensive than soldering and also is not conducive to automation.
It is the solution to these and other problems to which the present invention is directed.